Recently, products in the field of IT having built-in miniature digital cameras, such as mobile phones, smart phones, tablet PCs, laptops, etc., have been actively developed. Camera modules including digital cameras are required to have various functions, such as auto-focusing, alleviation of shutter shake, a zoom function, etc., and the recent trend in the development thereof is focused on increasing pixel count and miniaturization.
Conventional camera modules may include lens driving devices capable of performing an auto-focusing function and a hand shake compensation function. The lens driving devices may be constituted in various fashions, among which a voice coil unit motor is commonly used. The voice coil unit motor is operated by the electromagnetic interaction between a magnet secured to a housing and a coil unit wound around an outer peripheral surface of a bobbin, to which a lens barrel is coupled, thereby performing an auto-focusing function. An actuator module having such a voice coil motor is configured such that a vertically movable bobbin is capable of being moved reciprocatingly in a direction parallel to an optical axis direction while being resiliently supported by lower and upper elastic members.
IT products having conventional built-in miniature digital cameras include lens driving devices for setting a focal distance of a lens by adjusting the distance between an image sensor and a lens. However, conventional miniature digital cameras have a problem in that an auto-focusing time taken to perform an auto-focusing function is considerably long. Therefore, many efforts have been made to shorten the auto-focusing time, but the performance of the lens driving device may be deteriorated somewhat due to the unstable electromagnetic force and eccentricity of a lens barrel attributable to magnetic force.
Conventional camera modules may include a Hall sensor (not illustrated) and a sensing magnet (not illustrated), which are arranged to face each other in a direction perpendicular to the optical axis direction of a lens in order to detect the focal position of the lens. In this case, the Hall sensor senses a magnetic field of the sensing magnet and outputs a voltage corresponding thereto. The position of the lens in the optical axis direction may be detected using the voltage output from the Hall sensor, but the Hall sensor is incapable of accurately sensing the movement of the lens in the optical axis direction, and thus there is a limitation with respect to the ability to detect the position of the lens.